1. Field of the Invention
The present invention relates to a semiconductor laser and a method of manufacturing the same.
2. Related Background Art
Among conventional methods for manufacturing semiconductor lasers, the present invention relates to methods for manufacturing semiconductor lasers made on an In-P substrate and used for optical communication, more specifically, narrow radiation angle lasers that emit a laser beam with a narrow radiation angle. The narrow radiation angle laser is classified into a type having an active layer and a waveguide layer, which make up the laser, with film thicknesses varying along the cavity direction for narrowing a radiation angle, and a type having a taper shaped active layer stripe with the width varying in the cavity direction to have a taper shape for narrowing a radiation angle, without varying the film thicknesses of an active layer and a waveguide layer. In the latter type, the taper shaped active layer stripe is formed by etching laminated films including the active layer formed on a substrate through a taper shaped dielectric mask provided on the laminated films.
The following describes a method of manufacturing the above-mentioned narrow radiation angle laser of the taper shaped active layer type, with reference to FIG. 6. FIG. 6 is a front view of the semiconductor laser at the output end side of the semiconductor laser. Firstly, as shown in FIG. 6A, a n-InP layer 601, an active layer 602, and a p-InP layer 603 are formed on a n-InP substrate 600 in this order. On these layers, a SiO2 film 604 is deposited, as shown in FIG. 6B. Next, the SiO2 film 604 is patterned in a stripe shape by ordinary photolithography and reactive ion etching methods, resulting in a SiO2 mask 605 as shown in FIG. 6C. The SiO2 mask 605 is formed into a taper shape, so that the width of the mask is made narrower gradually, like a sector, from the rear end of the semiconductor laser toward the output end. The difference between the widths of the rear end and the output end of the SiO2 mask 605 approximately is 1 xcexcm. The cavity length is in a range between approximately 300 xcexcm and 800 xcexcm.
Next, the n-InP layer 601, the active layer 602, and the p-InP layer 603 are removed by a mixed solution including acetic acid, hydrochloric acid, and a hydrogen peroxide solution using the SiO2 mask 605 (See FIG. 6D). In this process, these layers are side-etched from the side edge portions of the SiO2 mask 605, so that the width of the active layer 602 becomes smaller than that of the SiO2 mask 605. This etching process is completed when the width of the active layer 602 becomes a predetermined value. A target value of the width of the active layer 602 is in a range between 0.5 xcexcm and 1.0 xcexcm. After that, in order to make the height of the upper surface of the n-InP layer 601, which is exposed as a result of the etching process, from the surface of the n-InP substrate 600 a predetermined value, etching is conducted by a mixture solution including hydrochloric acid and phosphoric acid so as not to change the width of the active layer 602. As a result of these wet-etching processes, a taper shaped active layer stripe is formed.
Next, as shown in FIG. 6E, a p-InP burying layer 606, a n-InP burying layer 607, a p-InP burying layer 608, and a contact layer 609 are grown in this order by the vapor growth method. Finally, as shown in FIG. 6F, isolation grooves 610 and 611 and a p-type electrode 612 are formed on the surface and a n-type electrode 613 is formed on the rear surface so as to complete the laser.
However, the above conventional manufacturing method has the following problems.
In the wet-etching process for forming a taper shaped active layer stripe, where the width of the active layer 602 is patterned into a sub-micron size by carrying out side-etching from the side edges of the SiO2 mask 605, the end point where the etching is to be finished (hereafter simply called xe2x80x9cetching end pointxe2x80x9d) is detected by measuring the width of the active layer 602 with an optical microscope so as to confirm that the width is within a predetermined dimension. In this case, it is difficult to measure the width of the active layer 602 with the optical microscope accurately. In addition, although the width of the taper shaped active layer stripe varies in the cavity direction, this stripe might be recognized as a stripe having a uniform width on the wafer, because the difference between the widths of the rear end and the output end thereof approximately is only 1 xcexcm. Therefore, it is considerably difficult to identify the output end portion of the stripe by a visual inspection.
Due to this difficulty, it might take considerable time to identify the output end portion, which deteriorates a production efficiency, or an error in identifying the output end position might occur. Furthermore, due to the difficulty in measuring the width of the active layer of a sub-micron size, improper measurement often occurs, which causes poor qualities of patterning and laser characteristics.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a semiconductor laser and a method of manufacturing the same, by which the etching end point can be detected easily and thus improper measurement can be prevented when patterning the width of an active layer stripe at the output end into a sub-micron size.
The semiconductor laser according to present invention has a basic construction composed of an active layer stripe including a first semiconductor layer, an active layer, and a second semiconductor layer that are laminated in that order on a substrate and are formed into a stripe-shape; a burying layer in which the active layer stripe is buried; and a contact layer formed on the burying layer.
To cope with the above-stated problems, the semiconductor laser having the basic construction of the present invention further includes a monitor stripe that is formed in parallel to the active layer stripe and is composed of the first semiconductor layer only at an output end of the laser, the monitor stripe is buried in the burying layer on which the contact layer is formed, and the active layer stripe and the monitor stripe are isolated electrically by an isolation groove.
With this construction, the etching end point can be detected easily by using a timing when the active layer in the monitor stripe at the output end disappears as a criterion. As a result, the width of the active layer stripe can be formed with high accuracy.
In the basic construction, it is preferable that the monitor stripe changes in width in a cavity direction so as to have a narrow width region at the output end and a wide width region provided at least at one portion of the stripe other than at the output end, and the wide width region of the monitor stripe is composed of the first semiconductor layer and the second semiconductor layer. With this construction, it becomes easy to identify the output end in the monitor stripe.
In the above construction, preferably, the first semiconductor layer of the monitor stripe in the narrow width region is thicker at a center portion in a width direction than the other portions.
In addition, in the above basic construction, it is preferable that the monitor stripe is composed of the first semiconductor layer, and the first semiconductor layer in the monitor stripe is thicker at a center portion in a width direction than the other portions.
The method of manufacturing a semiconductor laser according to the present invention includes: forming a lamination film for stripe by depositing a first semiconductor layer, an active layer including a semiconductor multilayer, and a second semiconductor layer in this order on a substrate; forming a mask by depositing a dielectric film on the lamination film for stripe and shaping the dielectric film into a predetermined shape so as to form a first dielectric mask; and etching the lamination film for stripe through the first dielectric mask so as to form an active layer stripe. In the step of forming the mask, a second dielectric mask also is formed in parallel to the first dielectric mask. In the step of etching the lamination film, the active layer stripe is formed through the first dielectric mask so as to be located just below the first dielectric mask and at the same time a monitor stripe is formed through the second dielectric mask so as to be located just below the second dielectric mask, and completion of the etching is detected according to a width of an active layer in the monitor stripe at an output end of the laser as a criterion.
With this method, the etching end point is detected according to the width of the active layer in the monitor stripe, and therefore an optimum condition for detecting the etching end point can be set for any construction of the active layer stripe.
In the above method, a width of the first dielectric mask and a width of the second dielectric mask may be set to be equal.
In addition, the second dielectric mask may be formed so as to vary in width in a cavity direction and have a narrow width region at the output end and a wide width region provided at least at one portion of the mask other than at the output end. With this method, it becomes easy to observe the width of the active layer in the monitor stripe at the output end.
In the above-stated method, preferably, assuming that at the output end of the laser subjected to the etching process a width of the active layer in the active layer stripe is Wa, a width of the first dielectric mask is W1, and a width of the second dielectric mask is W2, values of W1, W2, and Wa respectively are set so as to satisfy a relationship represented by the following Formula 1:
W2xe2x89xa6W1xe2x88x92Waxe2x80x83xe2x80x83(1)
With this method, the output end width of the active layer stripe located adjacent to the monitor stripe becomes the optimum value at the timing when the active layer just below the monitor stripe disappears.
Alternatively, the values of W1, W2, and Wa respectively are set so as to satisfy a relationship represented by the following Formula 2:
W1xe2x88x92Waxe2x88x92xcex1xe2x89xa6W2xe2x89xa6W1xe2x88x92Wa+xcex1xe2x80x83xe2x80x83(2)
where xcex1 denotes a value set according to a tolerance of Wa.
With this method, variations in the width of the active layer stripe further can be made smaller.
In the above-stated method, preferably, assuming that a width of the first dielectric mask at the output end is W1 and a width of the second dielectric mask in the wide width region is W2r, values of W1 and W2r respectively are set so as to satisfy a relationship represented by the following Formula 3:
W1xe2x89xa6W2rxe2x80x83xe2x80x83(3)
Even in this case, since the width of the second dielectric mask at the output end is narrow, it is easy to make visual inspection of disappearance of the active layer in the monitor stripe at the output end. Moreover, since the width of the second dielectric mask is wide enough at portions other than the output end, the second dielectric mask can be prevented from delaminating.
Furthermore, subsequent to the above-stated steps, the following steps further may be conducted: that is, forming a burying layer so as to bury the active layer stripe formed in the etching step; forming a contact layer on the burying layer; and forming an isolation groove at a position where the monitor stripe was formed so as to remove the monitor stripe.